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Remote Sensing Data Applications and Analyses
Prof. Turco, with Jens Redemann, designed a new approach for inferring the composition and origin of tropospheric aerosols using a combination of airborne lidar and sun-photometer observations, together with a multi-level radiative transfer analysis. Their results show that, while appropriate, coordinated data sets may be capable of achieving radiative "closure" in field experiments, current instruments and sampling strategies are probably not adequate for this purpose. Turning to satellite observations of stratospheric aerosols and gases, Helen Cox Steele showed that the information content of solar extinction-based techniques provide reasonable results for integrated quantities (such as total surface area, or volume), but offer much less information on size distribution. That analysis was extended to assess the accuracy of Stratospheric Aerosol and Gas Experiment (SAGE) ozone measurements, leading to a significant recalibration of previously published data. A similar approach was employed to analyze Polar Ozone and Aerosol Measurement (POAM) data. Using the POAM observations, the first Lagrangian analysis of polar stratospheric cloud evolution was carried out, where it was noted that existing cloud theories are not fully consistent with measurements.
Tropospheric Aerosols and Radiation: Currently, and in the future, enormous resources will be expended measuring particles in the atmosphere. The objectives ostensibly are to determine their chemical properties and impacts on radiation and climate. However, the usefulness of the measurements for these purposes is not yet clear. Prof. Turco and his students have assessed several aerosol observational systems in recent years to determine their practicality.
The characterization of tropospheric aerosols during the Pacific Exploratory Mission West (PEM-West) and Tropospheric Aerosol Radiative Forcing Observational eXperiment (TARFOX) field experiments was analyzed by Redemann using available data. In situ measurements from the PEM missions were employed to calculate equivalent aerosol backscatter coefficients, which were compared to backscatter coefficients derived from coincident lidar measurements. The analysis revealed substantial quantitative differences due to uncertainties in aerosol composition (and, hence, indices of refraction). Hence, a multi-layer inversion scheme, with synoptic characterization of sampled air masses via back-trajectory analysis (to identify likely aerosol type), was introduced, yielding a broadly consistent quantitative picture of the aerosols. For TARFOX, the data set is more complete. The approach used for PEM could therefore be extended to include the determination of the aerosol index of refraction within a set of distinct layers by using sunphotometer measurements to constrain the values. Then, forward calculations of the radiative energy balance, and evaluation of uncertainties and sensitivities in the resulting net fluxes, led to the important conclusion that radiative "closure," or energy balance, experiments have not been achieved to date. Further, doubt are raised that observational techniques currently available are adequate for this purpose.
Stratospheric Aerosols and Ozone: Prof. Turco and Helen Cox Steele have investigated the properties and information content of satellite-sensed aerosol data using precisely defined "surrogate" aerosols to carry out forward radiative modeling, and then comparing inversions of the resulting synthetic detector signals with the original aerosol properties. After optimizing the inversion, this process was repeated using satellite-based data. Focusing on limb extinction measurements, it was shown that such observations yield reasonable results for integrated quantities (for example, the total surface area or volume of the aerosol), but little information on size distribution. A thorough and critical analysis of the SAGE satellite limb extinction observations and data inversion scheme revealed that the retrieved aerosol and ozone parameters are largely insensitive to the number of instrumental wavelengths employed (beyond a few), and that the derived ozone values can be systematically biased when the aerosols vary realistically with time. Importantly, a systematic error was discovered in the extinction kernel used by the SAGE team, which subsequently led to the recalibration of the SAGE ozone archive--a crucial element in the global observational data base for ozone/climate assessments. Steele has also analyzed POAM satellite measurements, designing and applying a new retrieval algorithm, and used POAM measurements to analyze PSC evolution, as noted in the section describing research on PSC's.
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